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SPN 3364 FMI 18 — DEF concentration below target
CautionAftertreatmentLimp to the next safe stop
Applies to: Sae · Critical — contaminated DEF ruins SCR
DEF concentration below target.
Don't risk it — get a diesel tech on the truck.
Your SPN/FMI rides along, so nearby mechanics see the fault before they quote you.
How serious is it?
Caution. Limp to the next safe stop.
Most likely causes
- Diluted DEF (water added)
- Wrong fluid in tank
- Failed quality sensor
First moves (roadside)
- Refractometer test DEF (must be 32.5% urea)
- Drain and refill with known-good DEF if contaminated
Common misdiagnoses
- Replacing the quality sensor when DEF is actually bad
FAQ
What does SPN 3364 FMI 18 mean?
DEF concentration below target.
How serious is SPN 3364 FMI 18?
Caution — Limp to the next safe stop.
What's the most likely cause of SPN 3364 FMI 18?
The most common cause is: Diluted DEF (water added). Other possibilities include Wrong fluid in tank; Failed quality sensor.
Can I keep driving with SPN 3364 FMI 18?
Limp to the next safe stop. When in doubt, get a qualified mobile diesel tech on the truck before continuing.
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Find a mechanic or related guides
General diagnostic guidance for heavy-duty diesel engines (Cummins, Detroit, PACCAR, Volvo, Mack). Not a substitute for a scan-tool diagnosis on your specific truck.